1. Field of the Invention
The present invention in general concerns magnetic resonance tomography (MRT) as used in medicine for examination of patients. The present invention is particularly concerned with a spiral-coded method for accelerated MRT imaging as well as a magnetic resonance tomography apparatus which is suitable for implementation of such a method.
2. Description of the Prior Art
MRT is based on the physical phenomenon of magnetic resonance and has been successfully used as an imaging method for over 15 years in medicine and biophysics. In this examination modality the subject is exposed to a strong, constant magnetic field. The spins of the atoms in the subject, which were previously randomly oriented, thereby align. Radio-frequency energy can now excite these “ordered” spins to a specific oscillation. In MRT, this oscillation generates the actual measurement signal which is acquired by suitable reception coils. By the use of non-homogeneous magnetic fields generated by gradient coils, the measurement subject can be spatially coded in all three spatial directions. The method allows a free selection of the slice to be imaged, so slice images of the human body can be acquired in all directions. MRT as a slice image modality in medical diagnostics is distinguished predominantly as a “non-invasive” examination method via a versatile contrast possibility. Due to the excellent representation capability of the soft tissue, MRT has developed into a method superior in many ways to x-ray computed tomography (CT). MRT today is based on the application of spin echo and gradient echo sequences that enable an excellent image quality with measurement times on the order of minutes.
The continuous technical development of the components of MRT apparatuses and the introduction of faster imaging sequences has opened more fields of use for MRT in medicine. Real-time imaging for supporting minimally-invasive surgery, functional imaging in neurology and perfusion measurement in cardiology are only a few examples. In spite of the technical advances in the design of MRT apparatuses, the acquisition time of an MRT image remains the limiting factor for many applications of MRT in medical diagnostics. From a technical viewpoint (feasibility) and for reasons of patient protection (stimulation and tissue heating), a limit is set on a further increase of the performance of MRT apparatuses with regard to the acquisition time. In recent years various efforts have been made to further reduce the image measurement time by different approaches.
One approach to shorten the acquisition time is to reduce the quantity of the image data to be acquired. In order to obtain a complete image from such a reduced data set, either the missing data must be reconstructed with suitable algorithms or the incorrect image from the reduced data must be corrected.
The acquisition of the data in MRT occurs in what is known as k-space (frequency domain). The MRT image in what is known as the image domain is linked with the MRT data in k-space by Fourier transformation. The spatial coding of the subject, which spans k-space occurs by means of gradients in all three spatial directions. Differentiation is made between the slice selection gradient (establishes an acquisition slice in the subject, typically the z-axis), the frequency coding gradient (establishes a direction in the slice, typically the x-axis) and the phase coding gradient (determines the second dimension within the slice, typically the y-axis).
Depending on the combination or interleaving of the three gradients in the imaging sequence, the sampling of k-space can ensue in a Cartesian manner (thus line-by-line) or radially or in a spiral trajectory (path).
Spiral-shaped sampling of k-space, which represents a very efficient method, is exclusively considered in the context of the present invention. Spiral-shaped k-space trajectories were first proposed by Likes as possible alternatives to Cartesian sampling (U.S. Pat. No. 4,307,343). It was thereby shown that, in contrast to, for example, a Cartesian sampling, a spiral-shaped readout of the k-matrix leads to a more isotropic RF pulse response signal. In particular, the use of fast spiral sampling (fast spiral imaging) as an equivalent to echoplanar imaging (EPI) therefore increasingly gained in popularity, particularly in the fields of functional MRT, perfusion MRT, MR spectroscopy, diffusion MRT and phase contrast-based MRT flow measurements.
Image quality reductions due to frequency and phase errors during the readout times of the RF response signal are a previously unsolved problem in fast MRT imaging in general (fast single shot spiral scanning or fast multi shot spiral scanning and EPI). These reductions are manifested in EPI in the form of image distortions in the reconstructed image.
In fast spiral MRT imaging, the reconstructed image is locally fuzzy and blurred due to regionally-limited frequency shifts in k-space. In spiral imaging these errors are generally designated as “blurring” (in contrast to distortion, for example in Cartesian EPI sequences). The causes for these errors are primarily susceptibility limits and inhomogeneities in the tissue of the subject to be examined, such causes and therefore the errors are generally more strongly pronounced at higher field strengths.
The problem of “blurring” can be distinctly minimized when the readout time is shortened since relevant phase errors cannot develop as quickly or as strongly. Conventionally, this is achieved by reducing the number of the revolutions of the scanning path while retaining the size of the sampled region. There are approaches to utilize the parallel imaging technique (PPA technique) in spiral coding to shorten the readout duration. However, such a method is extremely computation-time-intensive and therefore is not practically applicable at the present point in time.